1. Field of the Invention
The present invention relates to an organic electro luminescent (EL) display panel, and more particularly to an organic EL display panel in which an aperture ratio is increased by improving a structure of a unit pixel.
2. Description of the Background
An organic EL display uses light emitted from an electrically excited organic light emitting diode OLED to display characters or images. Electrons, supplied from a cathode, and holes, supplied from an anode, recombine to excite the organic material. Generally, the organic light emitting diode OLED includes an anode electrode layer, an emitting layer (EML) for facilitating supply of electrons and holes, an electron transport layer (ETL), a hole transport layer (HTL) and a cathode electrode layer.
Methods for driving organic EL displays are generally classified as passive or active matrix methods. The active matrix method may include a voltage programming method and a current programming method, depending on the form of a signal that charges a voltage into a capacitor and maintains the charged voltage.
FIG. 1 shows an equivalent circuit of a conventional active matrix-type organic EL display driven by the voltage programming method. Referring to FIG. 1, pixels may be arranged in the form of a matrix defined by intersections among scan lines S1 to Sn, data lines D1 to Dm, and power lines V1 to Vm, and each pixel may include a switching thin film transistor ST, a driving thin film transistor DT, and a storage capacitor CST.
In each pixel, a source electrode, a gate electrode, and a drain electrode of the switching thin film transistor ST are coupled to data lines D1 to Dm, scan lines S1 to Sn, and a gate electrode of the driving thin film transistor DT, respectively. The storage capacitor CST is coupled between the drain electrode of the switching thin film transistor ST and a power lines Vn. A source electrode and a drain electrode of the driving thin film transistor DT are coupled to the power line Vn and an organic light emitting element OLED, respectively. The drain electrode of the driving thin film transistor DT may be electrically connected an anode electrode of the organic light emitting element OLED. A cathode electrode of the organic light emitting element OLED may be supplied with a common voltage for all pixels.
When the switching thin film transistor ST turns on from a selection signal applied to its gate electrode, a data voltage from the data lines D1 to Dm is applied to the gate electrode of the driving thin film transistor DT. Then, in response to a voltage VGS charged in the storage capacitor CST between the gate electrode and the source electrode of the driving thin film transistor DT, a current IOLED may flow through the organic light emitting element OLED via the driving thin film transistor DT, thereby emitting light from the organic light emitting element OLED.
In the voltage programming method as described above, a problem may arise in that brightness of an organic EL display panel may not be uniform due to deviation in characteristics of driving thin film transistors, such as threshold voltage or channel mobility.
Accordingly, complementary circuits for correcting this deviation in characteristics have been proposed. However, increasing the number of thin film transistors may decrease the pixel's aperture ratio.
On the other hand, assuming that a current source for supplying a current to pixel circuits is uniform for the entire panel, i.e., all data lines, the current programming type organic EL display may obtain uniform display characteristics even when the pixels' driving thin film transistors have a non-uniform voltage-current characteristic.
FIG. 2 is a pixel circuit showing a conventional current programming method for driving an organic EL display, where a single pixel is shown. Referring to FIG. 2, a driving thin film transistor DT is coupled to an organic light emitting element OLED to supply a current for emitting light, and a data current IDATA, which is applied through a switching thin film transistor ST1, controls the amount of current flowing through the driving thin film transistor DT.
When switching thin film transistors ST1 and ST2 are turned on by a selection signal from a scan line Sn, the driving thin film transistor DT is diode connected. Consequently, a storage capacitor CST is charged to a voltage as a current flows through it. Namely, a potential of a gate electrode of the driving thin film transistor DT drops, thereby causing the current to flow from a source electrode of the driving thin film transistor DT to a drain electrode of the driving thin film transistor DT, so that the storage capacitor CST is charged to the voltage corresponding to the data current IDATA for setting brightness. Next, the switching thin film transistors ST1 and ST2 are turned off, and a thin film transistor ET, which is coupled to an emission control line En, is turned on. Then, power is supplied from a power supply line VDD, and a current IOLED corresponding to the charged voltage of the storage capacitor CST flows through the organic light emitting element OLED to emit light with a preset brightness.
However, since the current IOLED flowing through the organic light emitting element OLED may be minute, and the voltage range of the data line Dm may be wide, it may take a relatively long time to charge a parasite capacitor of the data line.
Additionally, increasing the number of thin film transistors disposed in a unit pixel may significantly reduce the aperture ratio, which may deteriorate brightness. Furthermore, the display's lifetime may be reduced if the pixel circuits are driven with a high current.